1. Field of the Invention
The present invention relates to an organic light emitting display device and a method of fabricating the same.
2. Description of the Related Art
An organic light emitting (or organic electroluminescence) display device is a type of flat panel display device. In more detail, the organic light emitting display device is an emissive display device that electrically excites an organic compound in order to emit light. Organic light emitting display devices do not need a backlight as is used in liquid crystal displays (LCDs), and thus may be made to be relatively lightweight and slim, and have a relatively simple structure. Also, organic light emitting display devices may be fabricated at relatively low temperature and have characteristics such as a relatively high response time of 1 ms or less, a relatively low power consumption, a relatively wide viewing angle, and a relatively high contrast.
Organic light emitting display devices include an organic emission layer between an anode and a cathode, and holes supplied from the anode and electrons supplied from the cathode combine in the organic emission layer to form excitons which transition or decay (e.g., from an excited state or a ground state) to emit light.
Organic light emitting display devices can be classified into a bottom emission type and a top emission type according to a direction in which light generated in the organic emission layer is emitted. If an organic light emitting display device including a pixel driving circuit is the bottom emission type, an aperture ratio is limited because the pixel driving circuit occupies a large area of a substrate of the organic light emitting display device. Thus, the top emission type organic light emitting display device is introduced to improve the aperture ratio.
FIG. 1 is a cross-sectional view illustrating a structure of a conventional top emission type organic light emitting display device. Referring to FIG. 1, a buffer layer 110 is formed on a substrate 100 formed of glass or plastic. A thin film transistor is formed on the buffer layer 110 and includes a semiconductor layer 120 having source and drain regions 120a and 120c and a channel region 120b between the source and drain regions 120a and 120c, a gate insulating layer 130, and a gate electrode 140.
An interlayer insulating layer 150 is formed on the substrate 100 and on the thin film transistor. Then, contact holes 155 exposing parts of the source and drain regions 120a and 120c are formed in the interlayer insulating layer 150 and the gate insulating layer 130.
Source and drain electrodes 160a and 160b electrically connected with the source and drain regions 120a and 120c through the contact holes 155 are formed, and a planarization layer 170 is formed on the substrate 100 and on the source and drain electrodes 160a and 160b. 
A via hole 175 exposing a part of the drain electrode 160b is formed in the planarization layer 170. A first electrode 180 contacting the drain electrode 160a through the via hole 175 is formed on the substrate 100 and on the planarization layer 170. The first electrode 180 may include a reflective metal layer 180a and a transparent conductive layer 180b such as an ITO layer formed on the reflective metal layer 180a. 
In addition, a pixel defining layer 190 is formed on the first electrode 180. The pixel defining layer 190 is formed to a thickness from about 0.5 to 1 μm using an organic material (only), and then patterned to include an opening 200 for exposing the first electrode 180.
An organic layer is formed in the opening 200. The organic layer includes at least an organic emission layer, and may further include at least one of a hole injection layer, a hole transport layer, an electron transport layer, or an electron injection layer.
A second electrode is formed on the substrate 100 and on the organic layer to complete the formation of the top emission type organic light emitting display device display.
One method of fabricating the organic layer is to use a laser induced thermal imaging (LITI) method. When the organic layer is formed by the LITI method, if the pixel defining layer is formed to a thickness from about 0.5 to 1 μm as described above, there is a large height difference between the pixel defining layer and the first electrode, and thus the opening of the first electrode and a transfer layer of a donor substrate do not closely contact each other. Consequently, transfer energy becomes high, which may stimulate degradation of the organic layer, and the organic layer may not be properly transferred onto an edge part of the opening, which may result in an open defect. Thus, it is necessary to reduce the height difference between the pixel defining layer and the first electrode.
FIG. 2 is a photograph of an area surrounding a via hole after a pixel defining layer is formed to a thickness of 2000 Å using an organic material.
Referring to FIG. 2, in order to increase efficiency in fabricating an organic layer using an LITI method, a thin pixel defining layer was formed to a thickness of about 2000 Å using an organic material, for example, polyimide. The organic material, as illustrated with reference mark A, has a relatively good ability to fill the via hole. However, in order to form the pixel defining layer thinly, the pixel defining layer is formed by spin coating, and thus uniformity or dispersion is poor. As a result, an open defect may result around the via hole as illustrated with reference mark B. In addition, a protruding edge of a first electrode may not be fully covered, and may cause a short circuit between the first electrode and a second electrode. Moreover, since the pixel defining layer formed of the organic material does not have a rigid layer characteristic, it may tear during removal of a transfer layer of a donor substrate after the organic layer is formed on an opening of the first electrode. Thus, there is an additional possibility of a short circuit between the first and second electrodes.
FIG. 3 is a photograph of an area surrounding a via hole after a pixel defining layer is formed to a thickness of about 1000 Å using an inorganic material.
To overcome (or solve) the problems of the organic layer of FIG. 2, the pixel defining layer can be formed of an inorganic material, such as silicon nitride, that does not tear during removal of the transfer layer of the donor substrate after the organic layer is transferred due to its relatively high rigidity, and may be formed thinner than the organic layer of FIG. 2. However, as illustrated with reference mark C, the inorganic material has a poor ability to fill an inner via hole. Also, if the layer is increased in thickness, cracks may occur around the via hole (reference mark D) and a protruding edge of the first electrode due to stress, and thus a short circuit between the first and second electrodes may still occur.